This invention relates to a focus detecting device capable of providing range detection signals corresponding to the distances between the camera and objects by utilizing the principle of the coincident type range finder. More particularly, it concerns a focus detecting device effective even when the brightness of the objects is insufficient.
There has been proposed a focus detecting device which provides a range detection signal by photoelectrically detecting the coincidence of images transmitted through a fixed light passage and a movable light passage by employing the principle of the coincidence type range finder.
An illuminating method combined with the described focus detecting device also has been proposed for additionally illuminating objects by an illuminating means, such as a lamp, when the photoelectric current of the photoelectric elements reduces due to insufficient brightness of the object being photographed.
However, the conventional illuminating light source for additionally illuminating the object when the brightness of the object is insufficient, requires a large amount of energy to secure the effective function of the individual photoelectric elements and consumes a large quantity of electricity because the light receiving area of the photoelectric elements is fixed. Further, the conventional illuminating system requires an additional space as the optical systems of the illuminating system and the range detecting system are separately provided and consumes wasteful electricity as the illuminating system illuminates useless area other than the object area.